A noncontact type vacuum pump equipped with claw rotors is an example of a two-shaft rotary pump. For example, in an exhaust structure of a claw pump and a method of exhausting a gas which were proposed by the applicant of the present application, the pump comprises: a cylinder forming a pump chamber; one side plate and the other side plate covering end faces of the cylinder; two rotating shafts being arranged in parallel in the cylinder, the rotating shafts being rotated in opposite directions; two rotors being respectively integrated with the two rotating shafts, the rotors having hook-shaped claws just as meshed with each other in a noncontact manner, so as to compress a sucked gas; a rotary drive unit; a gas inlet being communicated to a part of the pump chamber where the gas in the cylinder is not compressed; and gas outlets being provided in the both side plates and opened in a part of the pump chamber where the gas in the cylinder is compressed (see Patent Document 1). With this structure, gas exhausting efficiency and function of the claw pump can be improved.
In case that a conventional two-shaft rotary pump, e.g., claw pump, is applied to a multistage pump, both ends of a rotating shaft, on which a plurality of rotors are arranged in the axial direction, are supported by bearings in a manner such that the rotors are sandwiched between two bearings (see Patent Document 2). With this structure, compression ratio of a gas can be increased by using a plurality of the rotors (multistage rotors) and the multistage cylinders, but heat is generated in each of the multistage cylinders by compressing the gas, so each of the rotors thermally expands. Further, by providing the multistage rotors to the one rotating shaft, side clearances between the rotors and end wall portions of the cylinders are badly influenced by total thermal expansion of the rotors. Namely, by the total thermal expansion of the rotors, it is difficult to suppress gas leakage by reducing the side clearances, so performance of the pump cannot be improved.
In the claw pump, the sucked gas (air) is compressed, in a compressing step, so as to improve gas exhausting efficiency. While ultimate operation of the rotary pump, no air is sucked and the pump theoretically conveys and compresses no air, so a workload of the pump is zero. However, even in the ultimate operation, air leaking from a small gap is sucked, and a non-opened space (a closed space) has negative pressure, when the non-opened space formed by the rotor and the cylinder is communicated to the outside (a space whose pressure is higher than pressure of air discharged from the pump) via the gas outlet, so the exhaust air flows backward into the pump. The air which has flown backward into the pump is recompressed and discharged to the outside again. Namely, an unnecessary process is performed, so a power load must be enlarged and temperature in the pump must be increased. Note that, the ultimate operation is operation at ultimate pressure, and the ultimate pressure is a pressure generated by the pump, in a state where a gas inlet of a vacuum pump is closed (amount of the exhaust air is zero), with the maximum capability for producing a vacuum condition.
Namely, the power load of the pump is enlarged and operating efficiency thereof is lowered by the exhaust air flowing backward into the pump while the ultimate operation, etc. By the exhaust air flowing backward, the temperature in the pump is increased, so contact of the rotors and deterioration of important parts, e.g., oil seal, bearing, are caused by the thermal expansion, so reliability of the pump must be lowered. Thus, by merely reducing the compressed space capacity immediately before exhaust opening so as to suppress the amount of the air flowing backward, a side opened to the air, in which an amount of the exhaust air is large (in case that the pressure of the sucked air is close to atmospheric pressure), is excessively compressed. Further, reduction of a flow amount will be occurred by reduction of the capacity of the pump. Note that, the backward flow of the gas is sure to be occurred as far as the capacity immediately before exhaust opening exists, so there is a problem that the above described problems must be reasonably de-escalated. Conventionally, the problems have been solved by imposing a prescribed restriction on operating conditions, so it is difficult to improve operating efficiency.
Note that, the applicant of the present application has proposed a rotary type vacuum pump having vanes (vane pump). The vacuum pump has a gas outlet, and a first check valve is provided in the gas outlet. Further, a pressure escape hole for making the compressed gas in the vacuum pump, whose pressure is higher than the outside air pressure, escape to the outside air and reducing a power loss of the vacuum pump is formed, and a second check valve is provided to the escape hole. The gas outlet and the escape hole constitute a gas discharge hole of the vacuum pump (see Patent Document 3).
With this structure, the escape hole is formed in a circumferential wall portion of wall portions constituting the cylinder, so that temperature rise can be suppressed even if excessive compression occurs in the pump.